Chain branching is not as common with anionic polymerization as it is with free-radical polymerization and cationic polymerization.

1. Propose a mechanism for chain branching in the polymerization of acrylonitrile.
2. Compare the relative stabilities of the intermediates in this mechanism with those you drew for chain branching in the cationic polymerization of styrene (Problem 26-6). Explain why chain branching is less common in this anionic polymerization.

Anionic polymerization is a type of chain-growth polymerization in which an anionic initiator transfers a charge to a vinyl monomer which then becomes reactive. In polymerization of acrylonitrile, the carbanion from growing polyacrylonitrile chain abstracts the hydrogen from middle of a polyacrylonitrile chain and forms another carbanion which is stable by inductive effect of cyanide group. This carbanion attacks at the double bond of acrylonitrile which leads to branching in polymer chain. Carbanion stability enabled the branching in the polymer chain.

Mechanism for the branching in polymerisation of acrylonitrile

The chain branching hydride transfer (from a cationic mechanism) or proton transfer (from an anionic mechanism) ends a less-highly substituted end of a chain and generates an intermediate on a more highly substituted middle of a chain (a tertiary carbon in these mechanisms). This stabilizes a carbocation, but greater substitution destabilizes a carbanion. Branching can and does happen in anionic mechanisms, but it is less likely than in cationic mechanisms.